For an automatic transmission which transmits an engine output via a torque converter, a neutral idle control (hereinafter, simply referred to as “the neutral control”) for automatically bringing the automatic transmission into a neutral state by controlling a frictional engageable element for a starting gear position to a disengaged state while a vehicle is stopped within a D-range (drive range or forward drive range), is known. When executing the neutral control, a turbine of the torque converter rotates following an engine operation at no load or an extremely low load; therefore, compared to the stopped state within a first gear range in which the turbine is fixed, the load of the torque converter and an engine load are reduced and fuel efficiency of the engine improves.
Further, as an art for reducing the burden on a vehicle driver while the vehicle is stopped due to traffic congestion or waiting for a traffic signal, a brake hold control for automatically holding a braking force even when the driver moves his/her foot from a brake pedal upon satisfaction of a given condition in the stopped state is known. The brake hold control is released when a traveling start request is issued by, for example, the driver's accelerator operation, and once the braking force is thus released, the vehicle is able to start.
JP2008-167540A discloses one example of an art relating to the brake hold control. Note that JP2008-167540A discloses an art for reducing energy loss by reducing creep torque under the execution of the brake hold control.
Incidentally, the present inventors found an issue that a shock easily occurs depending on a condition when starting a vehicle from a stopped state where the neutral control and the brake hold control described above are executed. This issue is described in detail with reference to FIGS. 6 and 7.
FIG. 6 is a time chart illustrating a conventional example of changes in various elements over time in a case where a vehicle is started from a stopped state where the neutral control is executed but the brake hold control is not executed.
As illustrated in FIG. 6, in the stopped state before time t11, by the execution of the neutral control (Part (b) of FIG. 6), between first and second frictional engageable elements for a starting gear position, the second frictional engageable element is engaged (Part (e) of FIG. 6) and the first frictional engageable element is subjected to a slip control (Part (c) of FIG. 6), so as to bring the automatic transmission substantially into a neutral state (Part (f) of FIG. 6). Here, since the brake hold control is not executed (Part (j) of FIG. 6), a brake operation is being performed by the driver (Part (h) of FIG. 6).
When starting the vehicle in this state, first, the brake operation is released (time t11 in Part (h) of FIG. 6), and then the accelerator operation is started (time t13 in Part (i) of FIG. 6). When the brake operation is released at time t11, the neutral control is accordingly released (Part (b) of FIG. 6). Thus, a gear change from the neutral state to the starting gear range is started, and hydraulic pressure into an engaging hydraulic pressure chamber of the first frictional engageable element in a slipping state rises (Part (d) of FIG. 6).
When performing the gear change to the starting gear position in this manner, in the first frictional engageable element, a difference between a rotational speed of an input-side element connected to the turbine of the torque converter and a rotational speed of an output-side element connected to wheels of the vehicle gradually decreases and, eventually, the torque transmission between the two elements begins. Since an output rotational speed O10 of the automatic transmission is zero until the vehicle starts traveling, an inputted rotational speed of the automatic transmission (turbine speed T10 at Part (1) of FIG. 6) gradually decreases until the engagement of the first frictional engageable element is completed.
At time t12, once the torque transmission at the first frictional engageable element is started, the output rotational speed O10 of the automatic transmission starts to increase (Part (m) of FIG. 6), and the vehicle starts to creep (Part (g) of FIG. 6). Then, at time t14, the first frictional engageable element is completely engaged (Part (c) of FIG. 6) in a state where the output rotational speed O10 of the automatic transmission increases to a rotational speed which is high to some extent (Part (m) of FIG. 6).
Therefore, the turbine speed T10 decreases relatively slowly until the first frictional engageable element is completely engaged since the supply of hydraulic pressure into the engagement hydraulic pressure chamber of the first frictional engageable element is started. As a result, the first frictional engageable element is engaged in a state where an input-side inertia is relatively small, and therefore, the shock from the engagement does not easily occur (Part (n) of FIG. 6).
On the other hand, FIG. 7 is a time chart illustrating a conventional example of changes in various elements over time in a case where a vehicle is started from a stopped state where the neutral control and the brake hold control are both executed.
In this case, as illustrated in FIG. 7, in the stopped state before time t21, the brake hold control is executed (Part (j) of FIG. 7) and the brake operation has already been released (Part (h) of FIG. 7). In this stopped state, the start of the accelerator operation (time t21 in Part (i) of FIG. 7) triggers a vehicle start request.
When an accelerator operation is started at time t21, the neutral control is released (Part (b) of FIG. 7). Thus, a gear change from the neutral state to the starting gear position is started, and hydraulic pressure into the engaging hydraulic pressure chamber of the first frictional engageable element rises (Part (d) of FIG. 7).
Moreover, at time t21, although the brake hold control is released (Part (j) of FIG. 7), the braking force applied by a hydraulic brake, etc. is not immediately released, the vehicle does not immediately start (Part (g) of FIG. 7), and an output rotational speed O20 of the automatic transmission does not immediately increase (Part (m) of FIG. 7).
Since the output rotational speed O20 remains zero until the braking force is actually released at time t22 after the brake hold control is released at time t21, the reduction of a turbine speed T20 (Part (1) of FIG. 7) for engaging the first frictional engageable element is performed sharply. Therefore, the inertia due to the decrease in the rotational speed on the input side tends to become large.
Then, at time t23, since the first frictional engageable element is engaged in the state where the inertia on the input side is relatively large (Part (c) of FIG. 7), the shock from this engagement easily occurs (a reference character X20 in Part (n) of FIG. 7). Particularly, for example, when a centrifugal pendulum damper for reducing vibration caused by suspending an operation of a cylinder of the engine is provided on a drive source side of the automatic transmission, an inertial mass at the input side of the first frictional engageable element increases and, therefore, it becomes easier for the shock from the engagement to occur.